The present invention relates to combinations of stabilizers in polymer components of display elements to provide protection against the effects of heat, moisture, and oxygen. In a particular embodiment, the invention relates to the use of a combination of an acid scavenger compound, a hindered amine compound and a hindered phenol compound in cellulose acetate film for the fabrication of protective films for polarizers for use in display applications.
A variety of approaches have been disclosed for extending the useful life of polymeric objects under exposure to heat, moisture, and oxygen. U.S. Pat. Nos. 3,723,147 and 4,137,201 disclose the improvement of heat stability of cellulose ester polymers in regard to color and inherent viscosity under thermoplastic molding process conditions. The use of combinations of thioether compounds and epoxide compounds is recommended by U.S. Pat. No. 3,723,147. The use of combinations of cyclic phosphonite compounds with hindered phenol compounds and epoxide compounds is shown in U.S. Pat. No. 4,137,201. Both of these approaches are not successful in stabilizing polymers during long term exposure to humidity due the conversion of both thioethers and phosphonites to acidic products, which contribute to the deterioration of polymers.
Japanese Kokai JP 200172782 discloses the use hindered amine light stabilizer (HALS) compounds combined with UV absorbing compounds to reduce discoloration of cellulose ester film under high temperature and humidity when employed in protective sheets for polarizers in liquid crystal displays. Japanese Kokai JP06130226 discloses the use of antioxidant compounds for cellulose acetate, such as phenols or epoxy compounds in combination with hydroxybenzotriazole compounds to reduce discoloration of cellulose ester film under extended exposure to high temperature and humidity when employed in the polarizer of a liquid crystal display. U.S. Pat. No. 4,619,656 discloses the protection of gloss and yellowness index of polymer films against the action of UV light, heat, and humidity by the use of combinations of hindered amine light stabilizers, UV stabilizers, and phenol co-stabilizers. Japanese Kokai JP 05194788 discloses the reduced deterioration of dimensional stability, transparency, and bending strength of cellulose ester photographic film support during exposure to high temperature and humidity through the use of combinations of trimellitate ester plasticizers with hindered phenol compounds and hindered amine compounds or epoxide compounds. Although improved relative to unstabilized polymers, significant discoloration and crazing can still occur with these approaches.
It would be desirable to provide further enhancement in polymer stabilization against deterioration of both spectral and mechanical properties of polymer components of displays in confined environments under long-term use at elevated temperature and humidity.
In accordance with the invention, polymer film, coating, and molded article elements for use in a display device are described which comprise a polymer phase having molecularly dispersed therein an acid scavenger compound, a hindered amine light stabilizer compound, and a hindered phenol antioxidant compound, wherein the hindered amine light stabilizer compound and the hindered phenol antioxidant compound are present at a weight ratio of from 20:1 to 1:20, and the weight ratio of the acid scavenger compound to the total concentration of hindered amine light stabilizer compound and hindered phenol antioxidant compound is from 10:1 to 1:10. Use of a combination of an acid scavenger and a combination of two radical scavenger antioxidants dispersed in a polymer phase in accordance with the invention enables enhanced stabilization of the polymer when exposed to high heat and/or humidity condition, which has been found to be especially desirable when applied to polymer layers laminated into displays (e.g., polarizers in liquid crystal displays).